This invention relates to a method and apparatus for the analysis of anions or cations in a sample solution by ion chromatography.
The term "ion chromatography" designates the high-speed chromatography directed chiefly to inorganic ions which was published by H. Small et al in 1975 [Anal. Chem., 47, 1801 (1975)].
Ion chromatography has already been reduced to practice and has been finding extensive utility in applications to various forms of microanalysis such as analysis of ecological specimens and organic specimens, control and analysis of various processes, and elementary analysis.
FIG. 1 is an explanatory diagram illustrating the process flow in a conventional ion chromatograph adapted for the analysis of anions.
In FIG. 1, the ion chromatograph is seen to be provided with an eluant solution reservoir 1 for storing NaOH to be used as an eluant solution, a pump 2 for transferring the eluant solution of the reservoir 1 under pressure to a sample injection valve 3, which is adapted to collect a prescribed amount of the sample solution and transfer the collected sample solution with the eluant solution to a separation column 4 packed with a resin formed by electrostatically bonding an anion-exchange resin to a cation-exchange resin so as to function jointly as an anion-exchange resin and adapted to separate various ionic species from the influent liquid, a background separation column 5 (hereinafter referred to as BSC for short) packed with a strongly acidic cation-exchange resin and adapted to capture ions from the eluant solution, and a conductivity meter 6 adapted to introduce the liquid discharged from BSC 5 into the cell thereof and measure the conductivity of the liquid.
The problems encountered by the ion chromatograph of the construction described above originate in the BSC.
One of the problems is that under the ordinary conditions of analysis, the operation for regeneration of the BSC should be completed in 8 to 10 hours. The BSC is used herein for the purpose of capturing the ions present in the eluant solution, diminishing the background of such ions of the eluant solution on the conductivity meter, and enhancing the sensitivity of the detection of ions of interest. With the elapse of time, however, the BSC gradually loses its functioning ability. This is due to the reaction of the formula (1) shown below proceeding within the column and, consequently, the ion-exchange resin is converted from the H form to the Na form. EQU NaOH (Eluant solution)+Strong resin-H.sup.+ (BSC).fwdarw.Resin-Na+H.sub.2 O (1)
When the whole ion-exchange resin is converted to the Na form, the reaction of the formula (1) no longer continues. Consequently, the base line in the conductivity meter rises and, at the same time, the amplifying function upon the anions is lost. The conventional ion chromatograph, therefore, has been designed so that the function of the BSC is regenerated at fixed intervals by supplying of 1 N--3 N HCl to the BSC. Of course, the fixed intervals for this regeneration may possibly be shortened to 1 to 2 hours under such conditions of analysis which necessitate a highly concentrated eluant solution to be supplied in a high flow volume.
Another problem is that the peak form is impaired when the liquid eluted from the separation column is passed through the BSC. This decay of the peak form is ascribable to the fact that the BSC is formed by packing a tubule measuring 3 to 6 mm in inside diameter and 25 to 50 cm in length with an ion-exchange resin.
The prior art has been described with reference to an ion chromatograph designed for the analysis of anions. The conventional ion chromatograph used for the analysis of cations has substantially the same basic process flow and suffers similarly from the problems arising in the BSC.